Learn how to improve the repeatability of your measurements.

Shielding the Wires

To minimize measurement errors, ensure that your wires are properly shielded.

  1. Tie the grounds of the switch and the SMU together.
  2. Connect the shielded cables to their respective ground connections.

State of Charge

To ensure consistent and repeatable measurements, you must test cells at the same state of charge (SOC). Cell impedance is SOC dependent.

Rest Period between Measurements

Allow time for stabilization between measurements. A rest period improves consistency in consecutive electrochemical impedance spectroscopy (EIS) measurements or consecutive AC internal resistance (ACIR) measurements.

Allowing the cell to equilibrate between measurements minimizes changes due to electrochemical processes or heating effects. The time required to achieve partial or total stabilization varies depending on the following factors:

  • Specific cell chemistry
  • Stimulus applied to the cell
  • The state of health of the cell
  • Environmental factors, particularly temperature

State Changes

Avoid excessive state changes to improve measurement repeatability. The cell can change states because of electrochemical reactions or repeated measurements.

Electrochemical Reactions

Electrochemical reactions occur at the cell electrodes during EIS measurements. For example, charge cycles and discharge cycles can affect the electrode materials. Charge and discharge cycles can also affect the overall performance of the cell.

Repeated Measurements

Repeated measurements can contribute to cell degradation, especially if the cell is not in a stable or a well-maintained condition. Cell degradation can change the impedance characteristics over time.

Heating Effects

Balance the impact of local heading to improve measurement repeatability.

Electrochemical impedance spectroscopy (EIS) might induce local heating in the cell due to the applied AC signals. Local heating can alter the cell impedance characteristics between measurements. Minimizing the test current minimizes heating.

However, tiny test currents (for example, excitation currents) can result in increased measurement noise due to an insufficient signal-to-noise ratio. If the current is too low, the voltage responses might not be significant enough to accurately reflect the behavior of the battery at high frequencies.

Electrolyte Depletion

Electrolyte depletion occurs when the electrolyte in the battery cell is consumed over time, which affects the impedance.

Electrolyte components might deplete or change during measurements. The change in electrolyte components is relevant if the cell is subject to high currents or extended measurement periods.

Interface Changes

The solid electrolyte interphase (SEI) layer in lithium cells can change due to electrochemical reactions during measurements. The SEI layer impacts impedance and might evolve with repeated measurements.